For a hermetic package, expensive investigations have been made on maintaining characteristics and prolonging a lifetime. For example, a piezoelectric vibrator element is a sensitive element that is easily degraded through exposure to oxygen and moisture in its surrounding environment. In view of this, there has been an attempt to hermetically incorporate the piezoelectric vibrator element into a piezoelectric vibrator package, to thereby maintain the characteristics of the piezoelectric vibrator package and prolong the lifetime of the package.
As a hermetic structure of the piezoelectric vibrator package, the following hermetic structure has been investigated: a hermetic structure in which a glass substrate is arranged above an element base on which a piezoelectric vibrator element is arranged so that the glass substrate is spaced apart from and faces the element base, and under such state, the space between the glass substrate and the element base is sealed with a sealing material layer so that the piezoelectric vibrator element is surrounded by the sealing material layer. Ceramic, for example, alumina is generally used as the element base.
However, it has been known that the piezoelectric vibrator element has low heat resistance. Therefore, when the element base and the glass substrate are sealed with each other through firing in a temperature range in which the sealing material layer softens and flows, there is a risk in that the characteristics of the piezoelectric vibrator element are thermally degraded.